The "PDO minus AMO" composite precipitation anomaly maps prepared by Dr. Klaus Wolter of CIRES as part of an experimental seasonal forecast for the California DWR last year were consistent with the below average precipitation observed in California in the water years Oct 2012 - Sep 2013 and Oct 2013 - Sep 2014.

As referenced in Dr. Wolter's 2013 DWR presentation, Schubert et al. (J. Climate, 2009) found that five global climate models produced the least precipitation in the continental U.S. when the Pacific is cold (Pc) and the Atlantic warm (Aw). Conversely four of the five models produced the wettest conditions when the Pacific is warm (Pw) and Atlantic cold (Ac). As shown in this figure, all combinations with the Pacific cold (PcAw, PcAn, PcAc) produced below average precipitation and all combinations with the Pacific warm (PwAw, PwAn, PwAc) produced above average precipitation.

While the development of the 2014 El Nino has waxed and waned over the past several months, there has been a definitive change that might significantly impact the weather in California and the U.S. -- the Pacific has warmed. For five of the past six years the JUL-SEP PDO has been negative. This year the PDO has been positive since January.

To get an idea of how the change to a warm Pacific might affect precipitation in the U.S. the PDO index and AMO index values for JUL-SEP for the past 115 years were ranked and then divided into tercile classes, producing nine PDO/AMO states.

The PwAw composites show much more precipitation in Southern California and the Southwestern U.S. than the PcAw composites. It might be argued that it is the El Nino years in the PwAw composites that produce the wet signal in Southern California. However, if the El Nino years are removed from the PwAw composites for Oct-Sep and composites for Dec-Feb, a wet signal persists.

Of course there are many factors that can influence the amount of precipitation in a particular locale in a particular season or water year. Composites are not forecasts any more than monthly climate normals are forecasts; but both can provide useful guidance. In the PwAw case 7 of the 12 selected years in the composite were wet in Southern California and two-thirds of the years had near normal or above average precipitation. We'll see what happens in 2014-2015.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Downtown Los Angeles (USC) finished the water year (July 1 to June 30) with 6.08 inches of recorded rainfall. This is about 41% of the 1981-2010 normal of 14.93 inches. It was the seventh driest water year since recordkeeping began in 1877. When combined with last year's water year total of 5.85 inches, the back-to-back water years from July 1, 2012 to June 30, 2014 are the driest on record for Los Angeles. The total rainfall deficit over the two year period was about 18 inches.

El Nino has been knocking on the door, but until very recently the atmosphere has only been responding in spits and sputters.

The weekly OISST.v2 Nino 3.4 SST temperature anomaly has been vacillating in the range 0.4 to 0.6 beginning with the week centered on April 23, 2014 and now stands at 0.5. Nino 1+2 and Nino 3 anomalies have generally been increasing and as of the week centered on June 25 are at 1.6 and 1.0 respectively. Here is a plot of the Nino regions SST anomalies from the CPC. Following a period of strengthened tradewinds, another westerly wind burst has developed in the Western Pacific. There have been several WWB this year and the most recent WWB could lead to additional warming of equatorial SSTs.

The Global Wind Oscillation (GWO) can be a useful tool for evaluating the degree to which the atmosphere is responding to El Nino or La Nina conditions. As this plot of the GWO for the period March 1 to July 31, 1997 shows, during the spring and summer of 1997 El Nino conditions were already resulting in a definitive atmospheric response. Here is a plot of the GWO from March 1 to July 1 of this year. Over most of this period the GWO has shown a neutral or weak La Nina-like response. Recently the GWO has exhibited a positive shift in AAM anomaly that, if sustained, could be indicative of a coupled ocean-atmosphere response.

The Plume-based and Consensus Forecasts in the June 19 IRI/CPC ENSO Quick Look (PDF) show an increasing chance of El Nino conditions developing over the NH summer, with an approximately 80% chance of El Nino conditions being established by the OND season. A July 2 run of the CFSv2 forecasts Nino 3.4 anomalies to briefly decline, then increase substantially from July into October. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Beginning Wednesday evening (Feb 26) and continuing into Sunday (Mar 2), the storm systems produced the most rain over five days in Los Angeles since December 2010, ending a nearly 14 month period with record-setting dry weather. Los Angeles experienced the driest calendar year on record in 2013, and until Friday had recorded less water year rainfall than in 2006-07 — the driest water year (July 1 - June 30) since recordkeeping began in 1877.

According to preliminary precipitation data, Downtown Los Angeles (USC) recorded 4.52 inches of rain over the course of the storms, increasing its water year total from a dessicated 11% of normal to a not-too-bad-considering 50% of normal. Downtown Los Angeles' water year rain total now stands at 5.72 inches. This exceeds last year's cumulative precipitation total on this date by more than an inch, but still leaves us with a deficit of nearly six inches. The storms increased February's rainfall total to near normal, and jump-started March with nearly half its normal amount of rain. Prior to these storms the most rain recorded at Los Angeles in a day this water year was 0.29 inch back in November!

With this recent rainfall 2013-14 will not be the driest water year in Los Angeles, but one good storm, or even two, "does not a rain season make." In the short term these storms have dramatically reduced the fire danger, provided crucial relief to plants and animals, and increased groundwater and reservoir storage. What happens in the longer term we'll just have to see. Over the next several days a series of systems are forecast to produce additional rain from Central California north into the PNW. While no rain is forecast in Southern California over the next week or so, and the 8-14 day outlook is for below average precipitation, as long as the Pacific weather pattern remains progressive there should be additional opportunities for rain in the weeks ahead.

From July 1 to February 14 Downtown Los Angeles has recorded only 1.20 inches of rain. This is less than the 1.92 inches of rainfall recorded over the same period in 2006-2007, making 2013-2014 the driest water year to date since recordkeeping began in July of 1877. Downtown Los Angeles averages 14.93 inches of rain in a water year, July 1 through June 30.

Los Angeles recorded only a trace of rain in January, and so far this February has recorded only 0.23 inch. At the moment neither the GFS or ECMWF show any rain south of Pt. Conception through the morning of February 25. At that time both models have another high amplitude ridge over the West Coast, so unless something changes the chances for additional rain this month do not look good. The 1981-2010 normal rainfall for January is 3.12 inches and for February 3.80 inches.

Active weather in Central California culminating in a strong atmospheric river event February 8-9 produced double-digit precipitation totals in the coastal mountains and Sierra. Over the 5-day period ending Monday, February 10 at 3:15 pm precipitation totals as high as 23.51 inches were observed. Since February 12 another atmospheric river has been feeding moisture into Northern California and Oregon.

Since January 1 Downtown Los Angeles has recorded only 3.60 inches of rain, making it the driest calendar year since recordkeeping began in July of 1877. The previous record of 4.08 inches was set in 1953 and 1947. Downtown Los Angeles averages about 15 inches of rain in a calendar year.

Precipitation composites for years with comparable PD0-AMO indices constructed by Dr. Klaus Wolter of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado appears to have anticipated this year's drought in Southern California. While Oct-Dec anomalies for Southern California were -0.5 to -0.7 SD below the 1895-2000 Longterm Average, Jan-Mar anomalies were -0.1 to -0.3 SD below average. Assuming the anomaly maps to be correlative the drought's stranglehold on Southern California may weaken somewhat over the next three months.

To get an idea of how the precipitation anomaly might vary over the next three months, the US Climate Division Dataset Mapping Page was used to recreate the standardized Oct-Dec precipitation anomaly and Jan-Mar precipitation anomaly maps using the same years as Dr. Wolter's composites. Then standardized composite precipitation anomaly maps were constructed for the months of January, February, and March. Based on these composites some lessening of the severity of the drought in Southern California is suggested throughout the period Jan-Mar with the biggest improvement indicated in March -- except for coastal Southern California. A map showing the composite precipitation anomaly in inches for the period October to March was also generated.

While today's medium range forecasts and 6-10 day and 8-14 day precipitation outlooks aren't particularly encouraging there are some straws to grasp. The AO Index, which has been positive for most of the rainy season is now negative and the Global Wind Oscillation (GWO) is in its first full orbit into positive AAM territory in several months. Whether these changes eventually result in rain for Southern California we'll just have to see.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

According to Dr. Wolter the statistical forecast scheme integrates as many of the known influences on California's climate as possible, using data from 1950 onwards. In the absence of El Nino and La Nina influences considerations such as the status of the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), or recent Alaskan temperatures can be evaluated through statistical models to make a forecast.

Dr. Wolter also produced composite precipitation anomalies for the periods October-December and January-March for years with comparable PD0-AMO indices. In Southern California the Oct-Dec anomalies were -0.5 to -0.7 SD below the 1895-2000 Longterm Average and the Jan-Mar anomalies were -0.1 to -0.3 SD below average.

Since October 1 precipitation in California, Oregon and Washington has generally been well below normal. As of today the precipitation recorded at Downtown Los Angeles is at about 30% of normal for the water year (July 1-June30). If Los Angeles doesn't record more than 0.58 inch of rain over the remainder of December, 2013 will rank as the driest calendar year on record.

Updated November 5, 2013. My mistake -- a spreadsheet range error -- thanks for the heads up Reg! The driest January 1 to November 1 for Downtown Los Angeles was in 1972 with 0.92 inch. Here are the driest ten years for that period:

An energetic upper level low brought the first widespread precipitation of the rain season to Southern California October 9, with rain at the lower elevations and some snow in the local mountains. Rainfall amounts varied widely, ranging from a trace in some areas to over an inch in the mountains.

Downtown Los Angeles (USC) recorded only 0.04 inch for the storm, bringing the water year rainfall total to 0.13 inch, which is 0.31 inch below normal. Downtown Los Angeles has recorded only 2.76 inches of rain since January 1. This is one of the driest January 1 - October 20 in Los Angeles over the past 135 years! To get out of the bottom ten for calendar year rainfall Los Angeles needs about 3.5 inches of rain by December 31. Normal rainfall for November is 1.04 inches and for December is 2.33 inches.

For months I've been monitoring climate data and forecasts looking for something on which to base a 2013-14 Winter precipitation Outlook. Historically ENSO has played the major role in Southern California rain season weather, with El Nino conditions generally producing wetter weather and La Nina conditions generally drier. But ENSO conditions are currently Neutral and are expected to remain so through the end of the year.

Most climate models forecast slow warming of SSTs in the equatorial Pacific (NINO 3.4 region) over the next several months, but at this time of the year it would be very unusual to have substantial warming. The CPC/IRI ENSO Forecasts from IRI's October Quick Look indicate the probability of an El Nino developing before the end of the year is less than 20% -- and 20% seems high.

One computer model that at times has been forecasting above average precipitation in Southern California this Winter is the Climate Forecast System version 2 (CFSv2). The CFSv2 is fully coupled ocean-atmosphere-land-sea ice model used to forecast parameters such as sea surface temperature, temperature and precipitation rate. While skillful at predicting tropical SSTs, the CFSv2 generally performs very poorly when forecasting precipitation over land, so forecasts such as this earlier one for Dec-Jan-Feb must be viewed somewhat skeptically.

Another glass half-full observation is that the Madden-Julian Oscillation (MJO) has been relatively active this year and if this activity continues it provides recurring opportunities for enhanced U.S. West Coast precipitation. The downside is that it can result in periods of dry weather as well.

With the ocean and atmosphere neutral there's just not much on which to base a rain season forecast. As a result of the government shutdown the release of the official NOAA 2013-14 Winter Outlook has been delayed until November. The October CPC outlook is usually the basis of the initial official NOAA U.S. Winter Outlook. The U.S. Dec-Jan-Feb Precipitation Outlook, released October 17, calls for an equal chance of below average, average, or above average precipitation for all of California. We'll see!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

Western U.S. Precipitation - Percent of Average (WRCC)July 1, 2012 to June 30, 2013.

Downtown Los Angeles (USC) finished the water year (July 1 to June 30) with only 5.85 inches of recorded rainfall, making it the sixth driest water year since recordkeeping began in 1877. With a deficit of over 9 inches, the water year rainfall total was about 39% of the norm of 14.93 inches. This tabulation of Rain Season Totals from the NWS/Oxnard shows rainfall totals around the area ranged from a low of 14% of normal at Palmdale Airport to a high of 54% at Long Beach Airport and LAX.

As this WRCC precipitation map for the period July 1 to June 30 shows, below average precipitation was not limited to Southern California. If it were not for heavy precipitation related to atmospheric river events in late November/early December there would be even more red on the map.

It has been even drier since January 1. For example, since January 1 Downtown Los Angeles has received only 25% of normal rainfall; Burbank 21% of normal; LAX 30% of normal; Santa Barbara 28% of normal; and Palmdale 19% of normal. Surprisingly, rainfall totals for Los Angeles since January 1 are only the ninth driest on record. The driest January-June on record was 1972 when 0.26 inch was recorded. And guess what -- we went on to have a very wet rain season in 1972-73 with 21.26 inches of rain in Los Angeles!

As dry as it's been in the Los Angeles area since the first of the year, it has been drier at some locations in the San Francisco Bay Area. Remarkably, LAX has had more rain than SFO, and Burbank Airport in the San Fernando Valley has had more rain than Napa Airport in the Napa Valley!

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

If you were to only look at the Global Wind Oscillation (GWO) for November you might think our 2012-2013 El Nino hadn't flopped. The relative AAM anomaly peaked at about 1.4 around November 20, which is the highest it's been since the El Nino of 2009-2010.

Most of the AAM was added in the NH between 15°N and 30°N. This led to the development of a high amplitude mid-Pacific ridge, and undercutting of the ridge by the westerlies. This enabled a low near the dateline to tap tropical moisture and relay it into the circulation of a large eastern Pacific low. This linkage provided the moisture necessary to create the atmospheric rivers that produced excessive precipitation in Northern and Central California last week.

Over the 5-day period ending Monday morning (December 3) Northern and Central California recorded double-digit precipitation totals with several stations recording rainfall amounts in excess of 15 inches. Here are archived Public Information Statements issued by NWS San Francisco Bay Area/Monterey and NWS Eureka with some of the phenomenal precipitation totals recorded in those areas. This AHPS Precipitation Analysis shows the observed precipitation in the western U.S. for the 7-day period ending Monday morning.

New forecast methods developed by NOAA's ESRL PSD using satellite-derived and GFS medium range forecast data indicated high levels of water vapor transport early Friday and early Sunday. This typically characterizes AR events. This Evaluation of GFS Forecast Fields (PDF) shows the observed Integrated Water Vapor (IWV) for December 2, the corresponding analysis, and the 1-day to 5-day forecasts.

System dynamics were much weaker in Southern California, and the IWV content of the atmospheric rivers was less. (Friday IPW and Sunday IPW). Even so orographically favored areas were still able to wring several inches of rain from the moist flow. From Tuesday night to Monday morning Opids Camp recorded 3.02 inches, White Ledge Peak 4.09 inches, Refugio Pass 4.61 inches, and Rocky Butte 8.51 inches.

Over the same period Downtown Los Angeles (USC) recorded 1.03 inches, bringing the water year total to 1.36 inches. As of today that's 1.08 inch less than normal. Here's an archived copy of a NWS compilation of preliminary rainfall totals in our area.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

According to NWS Storm Reports a spotter in Simi Valley area near Tapo St. and the 118 Frwy reported 1.0 inch of rain in 15 minutes and 1.5 inches in 30 minutes with street flooding. A Mesonet station in Simi Valley recorded 0.84 inches of rain in 12 minutes. A Mesonet station near Pasadena recorded 1.1 inches in 25 minutes. Long Beach Airport set a new rainfall record for the date of 0.30 inches.

In a post last June I mentioned that the atmosphere wasn't responding to warming Pacific equatorial SSTs. At that time normalized relative AAM values had dropped to around -2 sigma. Although El Nino-like SST conditions developed in the equatorial Pacific in July and persisted in August, the atmospheric component (AAM) did not follow suit. The average AAM for the period July-September was less than any other El Nino year going back to 1950. (See chart below.)

Following a relative rapid 2 sigma increase over a period of six months, the July/August value of the Multivariate ENSO Index (MEI) dropped from 1.139 to 0.579, and its rank for the season dropped from just below a 'strong' El Nino to just below a 'weak' El Nino. Over the last month SST Anomaly in Nino Region 3.4 has dropped 0.6°C. A review of the ONI record reveals that a drop in the index, which is based on a 3 month running mean of Nino 3.4 anomaly, has not occurred before the Sep-Oct-Nov season during a warm episode.

As of mid September most models were still forecasting development of weak El Nino conditions. The IRI/CPC Plume-based ENSO Forecast puts the probability of El Nino conditions in the Sep-Oct-Nov season at a little over 80%! The probability of Neutral conditions is pegged at a little under 20%. The chance of returning to La Nina conditions is considered virtually nil. Going back to 1950, year two La Nina conditions almost always transition either back to La Nina conditions or to El Nino conditions. Depending on the climatology used there is either one (1985-86) or no cases of a transition to Neutral conditions from a second year La Nina.

Many dynamical model MJO forecasts are predicting a developing MJO signal in the Western Pacific, and that appears to be occurring. Velocity potential loops and Pacific Basin stitched satellite imagery show enhanced convection west of the date line, and today's MJO phase space plot from CAWCR/BOM shows a signal beginning to emerge. On the wind side, Mountain torques are over +4 sigma and Coriolis torque is at -2 sigma. Whether the (apparently) emerging MJO will help reboot our fading El Nino remains to be seen.

Update Friday, October 5, 2012. Eastward-propagating MJO-like signal didn't evolve as forecast by GFS (and several other models). Here's today's MJO phase space plot from CAWCR/BOM. Large swings in the magnitude of Mountain and Coriolis torques have continued. Relative AAM remains at about -1 sigma. The Early October CPC/IRI Consensus Probabilistic ENSO Forecast indicates an increasing chance of Neutral conditions developing over the next several months, but still gives an edge to El Nino conditions developing before the end of 2012.

Following is a chart comparing 2012-13 to warm ENSO episodes that have occurred since 1950. The warm episodes are based on the revised Oceanic Niño Index (ONI) based on multiple-centered 30 year base periods and are those specified in the CPC's tabulation of Cold & Warm Episodes by Season. A description of the parameters follows the chart. With the exception of years prior to 1957, a GWO phase space plot is included for each warm episode.

Jul-Sep AAM & Nov-Mar AAM: The mean of the global relative atmospheric angular momentum anomaly for the periods July 1 to September 30 amd November 1 to March 31 of the following year. Data is from the GWO phase space data file linked on the Global Synoptic Dynamic Model page of the PSD Map Room Climate Products. Reference Weickmann and Berry, 2008.

TAO/TRITON Time-Longitude Plot of SST and AnomalySaturday, June 30, 2012

Downtown Los Angeles (USC) will end the the 2011-2012 water year (July 1 to June 30) having recorded 8.69 inches of rain. This is about 58% of the 1981-2010 normal of 14.93 inches. The deficit of 6.24 inches is a little more than the 5-6 inch deficit recorded in a selection of similar second year La Nina years. According to data compiled by the NWS Santa Barbara will end the water year at about 66% of normal; Camarillo/Oxnard at 57%; Burbank Airport at 51%; LAX at 59%; and Long Beach Airport at 62%.

This TAO/Triton plot of Pacific equatorial SST and anomaly clearly depicts the evolution of our two year La Nina and the recent transition to warmer conditions. Is an El Nino in the works for this Winter? According to the Multivariate ENSO Index (MEI) a transition to El Nino conditions may already be underway. The April/May value of the MEI was +0.706. This is already within the range of a weak El Niño ranking. In his June 6 discussion of the MEI climatologist Klaus Wolter noted the last month's increase in the MEI was the 6th highest increase for this time of year since 1950. He also pointed out that it was the 4th monthly increase of this caliber in a row -- second only to the record of six consecutive large monthly increases in 1997 at the beginning of the mega El Nino of 1997-98. It will be very interesting to see if the string of large increases in the MEI continues with the May/June value.

While the ocean seems to be on board with the El Nino idea, the atmosphere appears to be balking -- at least for the moment. As of June 24, the AAM component of the GWO was down around -2.0, which is nearly as low as it's been during year two of the 2010-2012 La Nina. A positive value of AAM is generally associated with El Nino conditions.

More information about Southern California weather and climate can be found using our WEATHER LINKS page.

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